1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device suitably used for portable information equipment, notebook PCs, desktop monitors, television sets and the like, which require moving image display characteristics.
2. Description of the Related Art
Conventionally, liquid crystal display devices using a nematic liquid crystal material have been widely used as digit segment displays for watches, calculators and the like. In recent years, making full use of advantages of saving the space and consuming low power, liquid crystal display devices have come into wide use as displays for notebook PCs and desktop monitors. In particular, in the market of desktop monitors, it is no exaggeration to state that conventional CRT monitors have been increasingly replaced with LCD monitors. This trend has also spread to the market of TV monitors in which CRTs have long been a monopoly. Many manufacturers now actively make research and development with the aim of replacing CRT-TVs with LCD-TVs.
The hardest challenges to the spread of LCD-TVs are said to be the high-speed response characteristic capable of responding to moving images and the wide viewing angle characteristic independent of the angle at which an image is viewed. These are essential problems related to the electrooptic characteristics of liquid crystal materials, and various proposals have been made so far to tackle these challenges. As of today, however, no decisive solutions have yet been found. Under these circumstances, an optically compensated birefringence (OCB) mode has recently attracted attention as a mode capable of providing both the high-speed response characteristic and the wide viewing angle characteristic.
The OCB mode however has a very troublesome problem that initial transition from splay alignment to bend alignment is necessary before the device is driven. The splay-bend alignment transition has been analyzed in detail in the past, but the mechanism thereof has not yet been clarified.
The splay alignment and the bend (twist) alignment are topologically different in phase from each other. Therefore, change from splay alignment to bend alignment is possible only by shift of a disclination line separating these regions that are discontinuous from each other. In other words, a certain energy barrier exists to develop a bend-aligned region in a splay-aligned region. This energy barrier is considered comparatively low in places near spacers in the cell, flaws on alignment films probably produced during rubbing, places in which the alignment of liquid crystal molecules is disturbed for any reason, and the like. A bend-aligned region is said to develop and grow in the neighborhood of such a low energy-barrier place acting as a trigger (nucleus) when a voltage equal to or higher than a critical voltage (Vcr) at which the Gibbs free energy of the splay alignment is equal to that of the bend alignment is applied.
However, the transition to bend alignment (bend transition) brought by spacers, flaws, alignment disturbance and the like as described above largely depends on the probability, and therefore lacks in reliability and is poor in reproducibility. To ensure high-speed, reliable bend transition in all pixels of the entire panel, it is a requisite to form in advance at least one portion acting as the nucleus for the bend transition in each pixel.
For example, Japanese Laid-Open Patent Publication No. 11-7018 discloses a method in which high tilt-angle regions are partly provided in pixels to form regions bend-aligned in the initial stage or form regions that easily make the bend transition, and such regions are used as bend transition nuclei.
However, as discussed in the above publication No. 11-7018, to form high-tilt regions partly in pixels, it is necessary to form vertical alignment films partly after formation of horizontal alignment films, or perform phase separation using a special mixed alignment film material to form regions high in tilt angle and regions low in tilt angle. This considerably limits the kind of usable alignment film material. Also, if the method limits the usable liquid crystal material and alignment film material for formation of bend transition nuclei, it is not recommendable to adopt such a method from the standpoint of the fabrication process.
Japanese Laid-Open Patent Publication No. 9-96790, for example, discloses a method in which π-twist alignment is given as the initial alignment state by using a chiral agent-added liquid crystal material. In this technique, the π-twist alignment as the initial alignment continuously changes to pseudo-bend alignment only with application of a voltage, and thus the problem relating to the splay-bend transition can be avoided. The π-twist alignment may be virtually considered as uniaxial bend alignment as long as the voltage is high in the range of the drive voltage for a liquid crystal display device. The effect of flow effectively works in the relaxation time from a high voltage to a low voltage, as in pure bend alignment, and thus, the high-speed response characteristic of several msecs that is as fast as that obtained in the OCB mode can be obtained.
However, in the method described above, a large amount of a chiral agent must be added to attain the π-twist alignment as the initial alignment. It is necessary to add a chiral agent so that the ratio d/p is about 0.50 considering margins where d is the thickness of the liquid crystal cell and p is the natural chiral pitch of a chiral agent-added liquid crystal material. The addition of this amount of a chiral agent significantly degrades low-voltage optical characteristics, in particular, the transmittance characteristic at an azimuth angle of 45° under crossed-Nicols. This naturally degrades not only the optical characteristics of the liquid crystal panel observed from the front, but also the optical characteristics observed in slant directions, that is, the viewing angle characteristic.